The operating costs for desulfurization of high metal containing pertroleum oils are higher than economically feasible because of the rapid poisioning of the hydrodesulfurization catalyst. The high metals feeds mentioned here as characterized by Venezuelan stocks usually contain 100 to 400 parts per million of vanadium in the atmospheric residuum obtained from the crudes. The life of the desulfurization contact solids is limited by metals deposition in the pore structure of the solids. Attempts have been made to use high porosity catalysts as contact solids which are characteristically low in activity but do not produce the 0.5 to 1.0 weight percent sulfur fuel oils that the pollution regulations currently being promulgated require. On the other hand, high activity catalysts, used as the contact solids, which are effective in meeting these objectives have a very limited catalyst life because of blockage of the catalyst pores. It has been well known in the art, that contact with material such as bauxite is an effective means of removing vanadium from residual oils. Unfortunately, the reaction rate is quite low and the size of the pretreatment reactor becomes extremely large in regard to the catalytic reactor and thereby raises the capital cost of the facility to an uneonomic level.
Other previous work, mainly U.S. Pat. Nos. 2,987,467 and 3,151,060 also treat the metals containing petroleum stock by first stage hydrocracking. The method disclosed, however, is carried out at relatively high temperatures and results in much higher hydrogen consumption than the presently disclosed invention. Reduction of hydrogen consumption and improving hydrogen selectivity are very important economic parameters in the desulfurization of residual oils.
One of the great difficulties in the desulfurization of Venezuelan residuum is that the asphaltenic compounds contained in the resid are of a type that are difficult to desulfurize. In addition, the high vanadium content present in those asphaltenic structures acts as a contact solids poison which acts primarily by blocking up the pores near the external surface of the contact solids so that the internal surface becomes unavailable to carry out the desulfurization reaction. We have discovered a technique for pretreating the feedstock in a first stage prior to contacting it with a high active catalyst used as the contact solids, which allows the highly active catalyst to reach desulfurization levels of from 65 to 80 percent at reasonable space velocities and at a reasonable catalyst cost. Previously, all work on this kind of resid had to be carried out with a very porous catalyst which is quite amenable to poisoning by reaction of the metals with the active sites. Although it is simple enough to obtain 50 percent desulfurization of Venezuelan stocks, there is little economic interest in doing so. The new pollution laws being promulgated require at least 65 to 80 percent desulfurization to meet the specifications being placed on the fuel oils burned in metropolitan areas. These desired desulfurization levels can be reached economically by means of this invention.